1. Field of the Invention
This invention relates to the field of tensile strength members. More specifically, the invention comprises a method for affixing a termination to an end of a tensile strength member such as a cable.
2. Description of the Related Art
Tensile strength members must generally be connected to other components in order to be useful. A flexible cable provides a good example. The cable must generally include some type of end-fitting so that it can be transmit a load. For example, a cable used in a hoist generally includes a lifting hook on its free end. This lifting hook may be rigged to a load. The assembly of an end-fitting and the portion of the cable to which it is attached is generally called a “termination.”
A tough steel lifting hook is commonly attached to a wire rope to create a termination. A “spelter socket” is often used to create the termination. The “spelter socket” involves an expanding cavity within the end-fitting. A length of the wire rope is slipped into this cavity and the individual wires are splayed apart. A liquid potting compound is then introduced into the expanding cavity with the wires in place. The liquid potting compound transitions to a solid over time and thereby locks the wire rope into the cavity.
The potting compound used in a spelter socket is traditionally molten lead and—more recently—is more likely a high-strength epoxy. However, the term “potting compound” as used in this description means any substance which transitions from a liquid to a solid over time. Examples include molten lead, thermoplastics, UV-cure or thermoset resins (such as two-part polyesters or epoxies). Other examples include plasters, ceramics, and cements. The term “solid” is by no means limited to an ordered crystalline structure such as found in most metals. In the context of this invention, the term “solid” means a state in which the material does not flow significantly under the influence of gravity. Thus, a soft but stable wax is yet another example of such a solid.
The prior art approaches to adding a termination are explained in detail in commonly-owned U.S. Pat. No. 7,237,336, which is hereby incorporated by reference. An exemplary termination is shown in FIGS. 1-4. FIG. 1 shows a cable 10 made from advanced high-strength synthetic filaments. Many different materials are used for these filaments. These include DYNEEMA, SPECTRA, TECHNORA, TWARON, KEVLAR, VECTRAN, PBO, carbon fiber, and glass fiber (among many others). In general the individual filaments have a thickness that is less than that of human hair. The filaments are very strong in tension, but they are not very rigid. They also tend to have low surface friction. These facts make such synthetic filaments difficult to handle during the process of adding a termination and difficult to organize. The present invention is particularly applicable to terminations made of such high-strength filaments, for reasons which will be explained in the descriptive text to follow. While the invention could in theory be applied to older cable technologies—such as wire rope—it likely would offer little advantage and the additional time and expense of implementing the invention would not be worthwhile. Thus, the invention is not really applicable to wire rope and other similar cables made of very stiff elements.
Those skilled in the art will know that cables made from synthetic filaments have a wide variety of constructions. The example shown in FIG. 1 has a parallel core of filaments surrounded by a jacket of braided filaments. In other cases the cable may be braided throughout. In still other examples the cable construction may be: (1) an entirely parallel construction enclosed in a jacket made of different material, (2) a helical “twist” construction, or (3) a more complex construction of multiple helices, multiple braids, or some combination of helices and braids.
In the example of FIG. 1, the objective is to attach anchor 18 to the end of a tensile strength member in order to create a termination that can then transmit a load. In this example the particular tensile strength member is in fact a cable. Throughout this disclosure cables will be used as an example of a tensile strength member. However the invention should not be viewed as being limited to cables. The term “tensile strength member” or “tensile member” encompasses cables and sub-components of cables such as strands. The reader is referred to commonly-owned U.S. Pat. No. 8,371,015 for more detailed descriptions regarding the application of an attachment to a sub-component of a larger cable. The invention also encompasses non-cable structures intended to carry loads in tension.
Likewise, the term “anchor” should be viewed broadly to encompass virtually anything that can be attached to a cable. The anchor would ordinarily include some features facilitating attachment—such as a hook or threads. These features are conventional and have not been illustrated. Anchor 18 is instead depicted as a simple cylinder with a cavity 20 passing along its central axis.
FIG. 2 shows a sectional view through anchor 18 with the cable in position for securing to the anchor. A length of the cable has been passed through cavity 20. The reader will note that cavity 20 expands as one proceeds from the portion of the anchor facing the length of cable (the “proximal” end, which is the bottom end in the orientation of the view) toward the portion of the anchor facing in the opposite direction (the “distal” end, which is the top end in the orientation of the view). The expanding cavity in this example is a linear taper between two straight portions—all joined by fillets. Differing wall profiles may be used to create a wide variety of expanding cavities.
A portion of the cable filaments are separated to create splayed filaments 12. Liquid potting compound is then introduced into cavity 20 via a wide variety of methods. These include: (1) “painting” or otherwise wetting the filaments with potting compound and then sliding the anchor into position over the painted filaments, (2) positioning the splayed filaments in the cavity and then pouring in potting compound, (3) pre-wetting the filaments in a separate mold designed to wet the filaments, and (4) injecting pressurized potting compound into the cavity. However the potting compound is introduced, the splayed filaments remain within cavity 20 while the potting compound hardens. Once it has hardened the result is a mechanical interlock between the filament-reinforced “plug” of solid material and the cavity. Tension applied to the cable will be transferred to the anchor via the mechanical interference.
The anchor applied will usually be permanent. However, it is also possible to apply a removable anchor such as a two-piece or dissolvable design that in itself forms a sort of mold. This can then be removed and another anchor device attached to the “molded” composite section of filaments and solidified potting compound. As can be imagined by those skilled in the art, there are many ways in which this multi-step process could be devised to carry out the inventive method.
Of course, if enough tension is applied the termination will fail. Ideally failure would occur at 100% of the breaking stress of each individual termination. This would be a 100% efficient termination in which the termination hardware and method of termination did not detract from the performance potentially available in the filament material itself. In reality terminations fail below 100% of the filament strength and in some cases they fall far below it. FIGS. 3 and 4 serve to illustrate some of the reasons for this phenomenon.
FIG. 3 depicts a sectional view in which anchor 18 has been sectioned to reveal potted region 14 lying within the cavity in the anchor's interior. The cavity is defined by cavity wall 22—which is a profile revolved around central axis 24. It is not essential that the cavity be radially symmetric but most such cavities are radially symmetric. Proximal end 54 is the end of the anchor where the cable emerges. Distal end 56 is the opposite end.
The solid “plug” in potted region 14 may be conceptually divided into several regions. These are extended region 34, distal region 32, middle region 30, neck region 28, and transition region 26 (some terminations may be readily described using fewer regions and as few as only two—the distal region and the neck region). Transition region 26 represents the area where the freely-flexing filaments emerge from the potted region. Extended region 34 (which may not always be present) represents a region beyond the filaments that is 100% solidified potting compound. Distal region 32 represents the region containing filaments that is closest to the distal end of the anchor. The neck region contains filaments and is in the vicinity of the proximal end of the anchor. The behavior of these differing regions differs based on many factors, including: (1) the size of the cable, (2) the type of potting compound used, and (3) the temperature of the components during the transition of the potting compound to a solid.
FIG. 4 shows a depiction of filaments 38 as they lay locked within the solidified potting compound. This view illustrates one of the significant problems of the potting approach. Once the filaments are placed within the cavity in the anchor, it is very difficult to control their orientation with any specificity. The reader will note that the filaments are roughly arrayed about the anchor's central axis and roughly splayed into a fan. However, each individual filament tends to bend and slew in a random fashion. The random nature of this variance reduces the overall breaking strength of the termination and introduces variability in breaking strength from one termination to the next (since some will have better filament alignment than others).
The depiction of FIG. 4 shows only a few filaments for visual clarity. An actual cable may have several thousand to several million such filaments in the potted region. It is not possible to neatly arrange the filaments because there is no way to grip and hold them. One could conceptually improve the alignment by adding tension to the cable while the potting compound is still in a liquid state, but of course this action would simply pull the wetted filaments out of the anchor.
Another known problem is the difference in the filament-to-potting-compound ratio for different regions of the cavity. The distal extreme of the cavity tends to be rich in liquid potting compound and lean on filaments (liquid-rich region 40 in the view). The proximal extreme is just the opposite—packed with filaments with only a small amount of liquid compound seeping or wicking into the voids (liquid-lean region 42 in the view).
Most potting compounds are cross-linking polymers—such as epoxies. When the two constituents of such compounds are mixed an exothermic reaction is produced. The cross-linking rate is highly dependent upon temperature. To some extent the ultimate strength of the cross-linked solid is dependent upon temperature as well. Some heat is desirable but too much heat tends to produce short polymer-chain length.
Looking again at FIG. 4, those knowledgeable of exothermic reactions will perceive that the heating rate will vary within the potted region. In the liquid-rich region 40 the temperature will tend to rise more rapidly than in the liquid-lean region and the cross-linking will occur more rapidly (though the reader should note that for some potting compounds “rapid” may mean several hours up to a day or more). In the liquid-lean region 442 (typically the neck or transition regions), however, most of the volume is consumed by the filaments themselves. Only small “slivers” of potting compound are present and the heat of reaction in these slivers is largely absorbed in heating the filaments. Thus, the temperature in liquid-lean region rises slowly and the cross-linking process occurs slowly.
The local build-up of heat is not easily dissipated because the potting compounds and the filaments themselves tend to be good thermal insulators. This would not be true for a traditional cable made of wire filaments. Because steel is a good thermal conductor, traditional cables do not tend to create a significant temperature variation during the potting process.
Another phenomenon existing in the cure process is viscosity variation. This is particularly true for a cross-linking potting compound (though true to some extent for other compounds). When the liquid potting compound begins to heat up in a given area, its viscosity typically drops and it tends to ooze and fill voids more readily. In addition, the decreased viscosity allows the filaments to move more freely within the liquid potting compound. However, as the solid transition continues the viscosity rises and eventually rises a great deal. Thus, for many potting compounds, the viscosity at the initial stage will fall, then rise and solidification occurs.
The present invention seeks to exploit these existing phenomena and in some instances—where the phenomena do not arise naturally—the present invention seeks to create them.